When I use “open with” from Google Drive to open a PDF in MetaPDF, it just shows a blank screen with a rotating symbol and the message “Downloading file” forever.
There is currently some sort of bug with the Google Drive interactions with the version of MetaPDF hosted at metapdf.com. I suggest you use our built-in version instead: Go to the gear in the upper right --> Settings --> Browser Integration. Then set your PDF viewer to “Viewer with Annotations (beta)”.
If you do not use Paperpile, you can still download the file and open the downloaded file in MetaPDF, but that is admittedly not very convenient. We apologize for this inconvenience.